Aerospace lubricating fluids require the removal of contaminants by a system that is operable under harsh working conditions, e.g., such a system must be capable of withstanding extremes of temperature and sudden changes in pressure differential. In addition, a fluid clarification system for aerospace lubricating fluids must make efficient use of volume and weight, due to the economic and physical constraints associated with flight.
For example, aircraft power generators, commonly referred to as integrated drive generators, and aerospace hydraulic systems utilize oils for lubrication and motive power under highly demanding conditions. Lubricating oils include various forms of petroleum distillates, synthetic oils, shale and coal derived oils. Synthetic oils, e.g. ester, phosphate, silicone, glycol or polyphenyl ether based oil, are frequently utilized as lubricants where extreme operating conditions are encountered. Synthetic oils, particularly ester based oils, are frequently employed as lubricants in aerospace integrated drive generators where elevated temperatures and severe load conditions dictate the use of an oil capable of withstanding extended exposure to elevated temperatures. Ester based synthetic oils, for example, exhibit very high viscosity, high flash points, low volatility, and exceptionally low pour points compared with petroleum based oils, making them especially suitable for use in integrated drive generators. Aerospace hydraulic systems, where severe load conditions are encountered, often utilize phosphate based synthetic oils, wherein the oil acts as both the hydraulic fluid and the hydraulic system lubricant.
Lubricating oils used in such applications are frequently contaminated with water. Typically, it is such contamination that initiates corrosion processes, which lead to the breakdown of metal components unless controlled. For example, under the operating conditions of integrated drive generators, typically up to about 250.degree. F., water reacts with lubricating oils to chemically degrade the oil and increase corrosion rates of internal metal components. Water also hydrolyzes ester based synthetic oils forming alcohol and acid, which in turn lead to increased rates of corrosion.
The current solution to this critical safety problem is the implementation of frequent, and expensive, maintenance. Thus, time consuming, expensive, scheduled maintenance has become a necessity in maintaining these systems.
It is known that reducing the concentration of water in lubricating oils inhibits these chemical reactions, which reduces corrosion, thereby increasing the life of the mechanical parts, and increasing the time between oil changes. When the level of water is reduced to a low level, corrosion is inhibited, significantly extending the period for scheduled oil changes. The complete elimination of water from such lubricating systems inhibits corrosion completely.
No good solution to this problem has previously been available, indeed, it has been difficult to design a system which can remove liquid contaminants present in concentrations lower than about 500 parts per million (ppm) from a fluid environment in a simple, efficient, and cost effective manner. (As used herein, all parts per million are by weight.) For example, coalescing is an effective method for the removal of water from oils; however, coalescing is limited to the removal of only dispersed water and does not have the ability to remove dissolved water. Many lubricating oils can contain high levels of dissolved water which produce high rates of corrosion of the system components. Even at levels of about 1500 ppm, dissolved water in synthetic lubricating oils leads to an increased rate of corrosion. As the level of water falls below 500 ppm, the corrosion rate is reduced; and the corrosion rate is reduced significantly as the water level approaches 300 to 200 ppm. Accordingly, it is desirable to develop a system which can efficiently reduce the level of contaminants present in lubricating fluids to about 200 ppm or lower.
Systems designed for airborne use, as well as certain other applications, must also combine a high level of effectiveness with relatively light weight and a limited volume, i.e., a critical factor in the removal of contaminants from lubricating oils used in aerospace applications is the limit of weight and space imposed by fuel restrictions. Accordingly, a system for removing the water must ideally be both light weight and have a high adsorption capacity to volume ratio.
Particulate adsorbents such as kaolin, silica gel, anhydrous salts and molecular sieves, such as synthetic zeolites, can be used to adsorb contaminants from various fluid environments, e.g., water vapor in an atmospheric environment. Particulate adsorbents have a high surface area and can be selected for their high specific capacity for a particular contaminant. Particulate adsorbents such as synthetic zeolites also have a relatively low weight for a given volume of adsorbent. As such they are particularly applicable for use in aerospace where, as noted above, weight and volume are critical. However, direct contact of an adsorbent such as a synthetic zeolite with a contaminated liquid environment is often undesirable. For example, in the case of lubricating oil used in the aerospace industry, fine particles of the adsorbent may further contaminate the liquid and degrade mechanical components, requiring additional steps to remove the adsorbent particles. These lubricating systems, which are run at high revolutions per minute, generate an elevated level of vibration that in turn leads to the attrition of particulate adsorbents. Moreover, in the case of synthetic lubricating oils, adsorbents can remove beneficial oil additives--chemical additives in the oil intended to improve performance such as antioxidants, anti-wear agents and suspending agents--thus compromising oil performance. Direct contact of the adsorbent with the fluid is also inefficient since the capacity of the adsorbent is substantially reduced, and in some cases may be entirely lost.
Some of the problems associated with direct contact of adsorbents with the contaminated fluid may be overcome by separating the adsorbent from the fluid by means of a barrier. Prior art solutions have included fine mesh screens, fine denier fabrics and microporous membranes. While such porous physical barriers may prevent particles of the adsorbent from circulating in the fluid environment, they do not prevent the fluid environment from contacting the adsorbent. In addition, the barriers taught in the prior art do not prevent the migration of attrition particles into the bulk fluid, which may lead to fouling of the mechanical system components.
It is therefore desirable to provide a barrier which is permeable to contaminants in a fluid environment but not permeable to the bulk fluid. It is further desirable to use such a semipermeable barrier in conjunction with a particulate adsorbent for the removal of contaminants from fluid environments without necessitating further purification. Semipermeable, for purposes of this invention, is defined as a non-porous barrier that allows the passage of contaminants such as gases or water in a bulk fluid but is impermeable to the bulk fluid.